1. Field of the Invention
The invention relates to a process of cleaning food processing equipment, in particular membrane filters which are used for producing liquid foodstuffs, wherein the filters are contacted with an oxidator.
In the food industry, use is being made to an increasing extent of membrane filters, in particular plastic membranes such as polyvinylpyrrolidone, polysulphone, polyether-sulphone and certain types of polyamides, and ceramic membranes for removing undesirable insoluble matter from beverages and other liquids. Such membranes are also used for cleaning surface water. Such membranes ensure an expedient removal of undesirable constituents, in particular micro-organisms such as algae, fungi, yeast, and bacteria
The problem is, however, that such membrane filters become blocked even after a short time so that they become unusable. The blocked filters can be regenerated, for example by rinsing them through in the opposite direction. However, that is a complicated process and no longer effective in the long term because the contamination accumulates. In addition, it is difficult to remove some persistent organic contaminants in this way.
2. Description of Related Art
Enzymatic processes have been proposed for cleaning membranes. Thus, WO 98/45029 describes the use of cellulases and amylases for cleaning beer filtering membranes, after alkaline pretreatment of the membrane. Similarly, JP-A 4-267933 describes the use of proteinases and cellulases for cleaning separation membranes. These non-oxidative processes, however, are usually insufficiently effective in cleaning blocked membranes.
WO 97/45523 describes the use of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) as nitroxyl compound and hypochlorite and hypobromite as a reoxidator for cleaning beer-settling modules. The presence of halogen residues, especially bromine residues, is highly undesired in equipment used for preparing or treating beverages and other foodstuffs. Moreover, the amounts of oxidator and reoxidator used are high and therefore this method is relatively expensive.
WO 99/15256 discloses the use of cyclic nitroxyl compounds such as TEMPO together with a calcium-sequestering agent for cleaning filters to be used in purifying surface water. This method is not very effective for cleaning filters with heavy residues produced in the beverage industry.
U.S. Pat. No. 5,647,988 and JP-A 9-290141 disclose a method for cleaning ceramic membranes used in water-purification plants and the like, by using a back-washing method in combination with an oxidising agent such as sodium hypochlorite, chlorine and chlorine dioxide followed by a washing with a reducing agent.
Research has revealed that during the membrane separation process for cleaning foodstuffs such as beer, the membrane fouling starts with a complexation onto the membrane consisting of a complex of polyphenols and proteins. Subsequently other components such as (poly)saccharides and/or other hydrophilic residues of e.g. yeast adsorb onto the polyphenol protein complex and as a result the pores of the membranes are blocked. The blocking of the membrane pores results in reduced permeability and ultimately in an inefficient filtration process.
In order to clean the used membranes, it was surprisingly found that filters and other equipment used in the food and beverage industry can be effectively cleaned in a TEMPO-free process. The cleaning process is focused on targeting the polyphenol protein complex with an oxidising chemical aid such as sodium hypochlorite, Oxone, organic peracids or a transition metal complex together with peroxide compounds such as hydrogen peroxide or with hypohalous acids. In all cases it is not necessary that after the oxidative cleaning the membranes are treated with a reducing agent. The research has resulted in two embodiments, which are feasible for cleaning the membranes. The two embodiments of the method of the invention have in common, as the inventive concept, that the protein and/or polyphenol-containing residues are treated with the oxidising agent capable of oxidising the proteins and/or polyphenols, while minimising contact of the oxidising agent with the polysaccharides. The reduced contact with oxidising agent has the further advantage that the life cycle of the membranes is increased.